Conventional diaphragm pumps typically operate substantially at the surrounding or ambient pressure of the fluid being pumped, e.g. atmospheric pressure for air pumps. From the moment a conventional diaphragm pump begins operating, and especially where the pump output is connected to a load such as a bladder to be expanded or inflated, a back pressure develops within the pump chamber and outlet line. This back pressure tends to outwardly distend or "balloon" the pump's diaphragm. One disadvantage of ballooning diaphragms is excess wear necessitating more frequent replacement of diaphragms. Another disadvantage is the over-sized enclosures necessary to contain ballooned diaphragm pump cheers.
Some diaphragm pumps incorporate directional flow controllers or valves, such as that disclosed in my earlier U.S. Pat. No. 5,009,579. These flow controllers function to meter pump output among one or more outputs for example, the controllers may be used to alternately inflate bladders A and B of a bed pad or the like. Known controllers include a mating plate and rotor. Pump output is provided to the stationary plate by tubing and associated fittings and, as the rotor rotates, is directed among one or more output lines by an intricate etched pattern on the sealed mating surfaces of the plate and rotor. A vent may also be provided.
One disadvantage of these directional flow controllers is that the back pressure created in the output lines, as well as the output pressure of the diaphragm pump, tend to push the plate and rotor apart breaking their seal and thereby disrupting directional control of the pressurized fluid pump output. A related disadvantage is that since the rotor must be more strenuously biased against the plate to preserve the seal, the timer used to rotate the rotor must be capable of providing higher torque, and thus be of greater size and use more energy than would otherwise be necessary. Further disadvantages include the substantial cost of etching the plate and rotor, and the increased assembly and parts costs associated with such designs.